The present invention relates to an automatic frequency control apparatus, and more particularly to an automatic frequency control method and apparatus thereof which can be utilized in a receiver of a digital mobile communication system that applies a time-division multiplex communication system to a radio channel.
Mobile communication is a field that expects much demand in the future, and an automatic frequency control apparatus is absolutely required for the receiving-end thereof in order to prevent the deterioration of the receiving strength according to multi-path fading.
The circuitry of an automatic frequency control apparatus which constitutes the conventional receiving-end has a general analog-type phase-locked loop circuit, which complicates the circuitry of the receiving-end, and the implementation should be implemented in a discrete circuit. In other words, it is difficult for the conventional frequency automatic control apparatus to be implemented in a semiconductor integrated circuit.
Another conventional automatic frequency control apparatus which is not a discrete circuit type, is explained using FIG. 1 as follows:
Referring to FIG. 1, analog-to-digital converters (ADC) 101 and 102 convert the I-channel and Q-channel analog signals into digital form, while a carrier frequency deviation detector 103 detects a frequency deviation, i.e., a frequency error, using the I-channel signal and Q-channel signal which are converted into digital signals. The frequency error detected by a carrier frequency deviation detector 103 is changed into control voltage data of a voltage controlled oscillator that corresponds to the frequency error by a random walk filter 104, counter 105, logical product means 107 and a mode controller 106. A selector 108 selects and produces a predetermined initial value which is applied from carrier frequency deviation detector 103 when the automatic frequency controlling is initiated, and otherwise, selects to produce the output of counter 105. Thus, the output of selector 108 is converted into an analog-type control voltage by a digital-to-analog converter 109, and then is applied to a voltage controlled oscillator VCO, to thereby accomplish automatic frequency control. Here, the method of detecting frequency deviation by carrier frequency deviation detector 103 produces a compensation factor using the following expression (1) which is applicable only when the receiving signal accords to the .pi./4 QPSK method. ##EQU1## Here, .delta..omega. represents an angular frequency deviation, V.sub.I and V.sub.Q are the I-channel signal and Q-channel signal, respectively, T is a symbol cycle, and sgn represents a function that detects a sign(+ or -).
That is, an automatic frequency control apparatus shown in FIG. 1 has a defect in that its application field is limited because the frequency error is produced assuming that the phases of the I-channel and Q-channel mutually have regular relations (given that the modulator/demodulator method is .pi./4 QPSK). Here, too many additional elements are needed in constructing a receiving-end for an automatic frequency control apparatus, to thereby complicate the circuitry of a receiving-end.